Source : Webster's Revised Unabridged Dictionary (1913)

Diabolic \Di`a*bol"ic\, Diabolical \Di`a*bol"ic*al\, a. [L.
   diabolicus, Gr. ? devilish, slanderous: cf. F. diabolique.
   See {Devil}.]
   Pertaining to the devil; resembling, or appropriate, or
   appropriate to, the devil; devilish; infernal; impious;
   atrocious; nefarious; outrageously wicked; as, a diabolic or
   diabolical temper or act. ``Diabolic power.'' --Milton. ``The
   diabolical institution.'' --Motley. -- {Di`a*bol"ic*al*ly},
   adv. -- {Di`a*bol"ic*al*ness}, n.

Source : WordNet®

diabolical
     adj 1: showing the cunning or ingenuity or wickedness typical of a
            devil; "devilish schemes"; "the cold calculation and
            diabolic art of some statesmen"; "the diabolical
            expression on his face"; "a mephistophelian glint in
            his eye" [syn: {devilish}, {diabolic}, {mephistophelian},
             {mephistophelean}]
     2: extremely evil or cruel; expressive of cruelty or befitting
        hell; "something demonic in him--something that could be
        cruel"; "fires lit up a diabolic scene"; "diabolical
        sorcerers under the influence of devils"; "a fiendish
        despot"; "hellish torture"; "infernal instruments of war";
        "satanic cruelty"; "unholy grimaces" [syn: {demonic}, {diabolic},
         {fiendish}, {hellish}, {infernal}, {satanic}, {unholy}]